Securities regulators have sent subpoenas to hedge funds and other trading firms in a probe of possible insider trading before the U.S. government's long-term credit rating was cut last month, the Wall Street Journal said, citing people familiar with the matter.

Securities and Exchange Commission officials demanded more information about specific trades made shortly before Standard & Poor's downgraded the country's rating to AA-plus from AAA on Aug. 5, the paper said.

SEC officials are zeroing in on firms that bet the stock market would tumble, the Journal said.

It is unclear which investment firms are being investigated, but the subpoenas are unusually broad, seeking information about why certain trades were made, a person told the Journal. An SEC spokesman declined to comment to the Journal.

The SEC could not immediately be reached for comment by Reuters outside regular U.S. business hours.